Blood is usually donated into sterile plastic bags that contain anticoagulants. These bags (“blood bags”) are connected to one or more similar bags by plastic tubing in a closed system for maintaining sterility. After centrifugation of whole blood contained in a primary collection bag, plasma or plasma plus platelets can be separated from red blood cells in the bag: a higher centrifugal force can separate all cellular elements from the plasma, and a lower centrifugal force can separate the plasma plus platelets from the red cells; the plasma plus platelets can then be subjected to higher centrifugal force in order to separate the platelets from the plasma. Therefore, if separation of plasma, platelets, and red cells is required, a two step centrifugation is necessary, with a primary blood bag linked to two “satellite” bags in series. If separation of all cellular elements from plasma is required, a single-step centrifugation is necessary, with the primary blood bag linked to one satellite bag. In both cases, plasma will be contained in the last bag having transferred to this last blood bag via plastic tubing from the other bags.
Plasma is used frequently for transfusion to treat clotting disorders, to expand blood volume, to treat shock due to plasma loss in bums or hemorrhage. Plasma is also used frequently to prepare plasma substances, e.g., clotting factors, and other proteins like albumin. This process is referred to as plasma fractionation. The plasma used must not have excessive amounts of hemolysis, turbidity or bile pigments. Since donors are usually healthy, elevated bile pigments are not expected.
It is desirable to measure blood components, also referred to herein as analytes, that may be indicative, for example of disease state. These analytes may be determined in whole blood, serum, plasma, or in other solutions, for example buffer. In one such assay, red blood cells are separated from plasma by centrifugation, or red blood cells and various plasma proteins are separated from serum by clotting prior to centrifugation. Many tests conducted on plasma or serum samples employ a series of reactions which terminate after the generation of chromophores which facilitate detection by spectrophotometric measurements at one or two wavelengths. Elevated Hb in the blood, i.e., haemoglobinemia, can be due to disease states and as a result of specimen collection and handling. Elevated bile pigments can also be due to disease states. Increased lipid particles in the blood, also known as hyperlipidemia, can be due to disease states and dietary conditions. In blood banking, plasma containing certain undesirable or dangerous components will be discarded.
Although blood is screened for the presence of several viruses, there is no test which provides 100% assurance of the absence of these viruses, and there are still other harmful viruses which are never tested for. In order to increase assurance that harmful viruses are eradicated if present, viral inactivation processes are being developed. One method used for inactivating viruses in plasma is the addition of methylene blue (MB) to the plasma. Thus measurement of MB concentration may provide assurance that the plasma contains the required amount of MB.
Blood substitutes constitute another type of blood analytes. Blood substitutes are new products that are under development, for use instead of whole blood or red blood cells for transfusion. Most blood substitutes under development are made from human haemoglobin (Hb), but another type of blood substitute has been reported which is a milky-white emulsion containing tiny beads of perfluorocarbons wrapped in a surfactant. The former will create pseudohemolysis while the latter will create pseudolipemia, in serum and plasma specimens. Subunits of Hb-based blood substitute are chemically cross-linked for stability (cross-linked haemoglobin or CLHb) and produce absorbance spectra which are very similar to the absorbance spectra of normal Hb.
Blood transfusion is a life saving process performed after severe blood loss during trauma or surgery. Some advantages of using a blood substitute instead of blood or red blood cells are as follows: 1. blood substitutes are expected to be universally compatible with all blood types, therefore cross-matching will not be necessary; 2. maximum storage time of blood is 42 days, whereas blood substitutes could have a much longer shelf-life; 3. purification a blood substitute may include heat treatment, which may eliminate the threat of hazardous viruses such as HIV. However, a challenge blood substitutes will pose to the clinical laboratory is managing the effects of blood substitutes on blood tests. As described above, some blood substitutes will cause the appearance of pseudohaemolysis in serum or plasma specimens or will make these specimens appear as whole blood while other substitutes will cause the appearance of pseudolipemia.
Spectrophotometric measurement typically employs infrared (IR) or near infrared radiation (NIR) to assess the concentration of various constituents in a blood sample. Examples of photometric measurements using containers which hold a blood sample are disclosed in U.S. Pat. Nos. 5,291,884; 5,288,646; 5,066,859; and 5,366,903 (which are incorporated herein by reference).
U.S. Pat. No. 5,366,903 discloses a sampling device which allows photometric quantitative determination of an analyte in whole blood. The device overcomes the problems of having blood cells in a blood sample by effectively “squeezing out” red blood cells and providing a small volume of sample, free of red blood cell material, from which particular analytes can be measured.
Other applications of photometric methodology include non-invasive determinations of analyte concentrations such as described in U.S. Pat. Nos. 5,360,004; 5,353,790; and 5,351,685 (which are incorporated herein by reference). However none of these documents discloses a method of measuring blood analytes in a rapid fashion directly in the blood bag.
Current methods used for detecting haemoglobinemia, bilirubinemia, biliverdinemia and lipemia or turbidity utilize visual inspection of the specimen with or without comparison to a coloured chart. It is to be understood that those practising in the field use the terms lipemia and turbidity interchangeably. This is because lipemia is the major cause of turbidity in serum or plasma. In blood banking, turbidity is assessed by the ability to read print on a paper placed behind a plasma bag.
Screening of plasma specimens by visual inspection is semiquantitative at best, and highly subjective. Furthermore, visual inspection of plasma specimens is a time consuming, rate limiting process. Consequently, state-of-the-art blood analyzers in fully and semi-automated laboratories, and automated blood banking facilities cannot employ visual inspection of specimens.
Other methods to measure analytes employ direct spectrophotometric measurement of a diluted sample in a special cuvette. However, such methods are not rapid enough for screening samples. In order to obtain a measurement of the sample of the plasma or serum, specimen tubes must be uncapped, a direct sample of the specimen taken and diluted prior to measurement. Each of these steps is time-consuming and requires disposable cuvettes. In blood banking, sterile techniques must be practised; especially when blood products are not used promptly. Maintaining a closed system is necessary to avoid bacterial contamination, hence any screening for analytes must be performed with the bag-tubing system intact. Removing a segment of the tubing linking the blood/plasma bags by heat-sealing can be performed without altering the sterility of the blood products, but this too is time consuming. Therefore, a rapid and effective method for measuring analytes, including natural and non-natural compounds within plasma in the blood banking industry is required.
It is an object of the present invention to overcome disadvantages of the prior art. This object is met by a combination of the features of the main claims. The subclaims disclose further advantageous embodiments of the invention.